Fundamental Technologies

Galileo Spacecraft Pages

LEMMS Analog Electronics

Source: The Galileo Energetic Particles Detector, D. J. Williams et al., Space Science Reviews, 60, 383, 1992 (excerpts).

Figure 1-78 is a block diagram showing the LEMMS analog electronics. The unbuffered, preamp tail-pulse output signals are sent down the polytwist wiring harness to three LEMMS analog boards, where the signals are amplified and shaped. All signal processing in the LEMMS channels is done in the linear domain. Comparator-like discriminator circuits assigned to each of the eight channels fire when pre-defined threshold levels are exceeded.

Figure 1-78. LEMMS Subsystem Analog Processing

The resulting discriminator outputs are sent to the rate logic electronics, where combinatorial logic is used to define 32 rate channels (event bins). Logic definitions for these channels include requirements for coincidence of some discriminators and anti-coincidence for others; this is used to define desired events. Those events which fall into the rate channels (by having the correct combination of discriminator levels) are counted in custom APL-designed high rate 24-bit accumulator hybrids. The rate data is periodically read into the telemetry stream by the data system. Each channel preamplifier and amplifier electronics can be powered ON/OFF by command.

LEMMS data also are processed through a pulse height analyzer (PHA) that produces 13 46-channel energy spectra per major frame (13 x 1-2/3 s). The detector outputs analyzed are selected via command from detectors A, E1, and F1 with the normal selection being 11 spectra from detector A and one each from detectors E1 and F1.

Pulse Height Analyzer

Source:  Proposal for Jovian Orbiter Magnetospheric Particles Instrument (JOMPI ), Nov. 1976

The Pulse Height Analyzer consists of redundant digital voltmeters with input multiplexers and four independent peak detector circuits.  The bipolar (log compressed) input signal drives a peak detector hybrid which charges a capacitor positive to the peak input pulse value.  As the first input pulse crosses zero going negative the discriminator initiates a 2-second "wait" interval. A reset cycle then occurs and the peak detector is again ready to track positive signals.

The first pulse to arrive is always analyzed.  If the data system determines that a higher priority event has occurred, it generates a "Go" signal during the 2 microsecond "wait" period, and another measurement cycle begins.  The pulse inputs from Ja/Jb, Jc, K and L are analyzed within about 100 microseconds.

The digital voltmeter is a redundant 10-bit instrument.  It uses an internal voltage regulator and maintains 0.1% accuracy with 10% changes in input power. Much of the electronics is off until the measurement cycle.  This minimizes both power consumption and radiation sensitivity.

A self-calibration function is included in the PHA which is exercised as commanded by the experiment data system. This uses the internal voltage reference with precision resistor dividers to establish two points on the calibration. The calibration can verify operation of the system but will not show changes in the internal reference voltage since it will self-compensate. However, the two redundant voltmeters may be switched in and out for comparative checks if the data appear in error.

Next: Calibrations

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Updated 4/27/04, T. Hunt-Ward
tizby@ftecs.com